Glasses with extremely low absorption coefficients are available today for manufacturing optical fibers for data transmission and it is extremely valuable from the technical standpoint to know the basic absorption of such glasses, whose loss behavior can change when they are processed into optical fibers. Thus, a method is required which makes it possible to determine, as exactly as possible, extremely low optical absorption coefficients with typical values of 1 dB km.sup.-1 (2.3.10.sup.-6 cm.sup.-1). A similar problem arises in studying recombination processes in solids, wherein charge carriers are present, bonded to lattice defects or to impurity atoms incorporated in the lattice, the electronic excitation states of said carriers being occupied by resonant excitation with monochromatic light and can recombine emitting photons (radiant recombination or luminescence) and/or phonons (nonradient recombination). A serious problem here is the detection of the portion of the exciting-light power which is consumed in phonon emission, in order to determine the quantum yield of the luminescence radiation.
In both cases, a small amount of absorption must be determined as accurately as possible, either because the characteristic absorption coefficients are very low or because only thin-layer thicknesses of the materials under study are available and/or the charge carrier concentration is low.